1. Field of the Invention
The present invention relates to a variable valve device for internal combustion engines, and more particularly, to techniques for optimizing valve opening/closing timings of intake and exhaust valves.
2. Description of the Related Art
In recent years, more and more internal combustion engines (engines) have come to be equipped with cam phase variable mechanisms as a variable valve device for varying valve opening/closing timings (cam phases). Also, techniques have been developed whereby the cam phase variable mechanisms are applied to an engine having each cylinder provided with a plurality of intake valves and the valve opening/closing timings of all intake valves as well as only some of the intake valves are varied (split) in accordance with the operating condition of the engine (Japanese Laid-open Patent Publication No. 2009-144521).
Where the valve opening/closing timings of only some of the intake valves associated with the individual cylinders can be varied in this manner, the multiple intake valves can be made to open continuously as a whole, whereby highly flexible valve control can be executed so that the overall valve open period of the intake valves may be prolonged, making it possible to improve the operational performance of the engine.
As actuators for such cam phase variable mechanisms, hydraulic actuators operated by unit of operating oil pressure are generally used. Also in the aforementioned publication, a vane-type hydraulic actuator is used for an actuator for actuating all of the intake valves as well as for an actuator for actuating only some of the intake valves.
Where a plurality of hydraulic actuators are used, however, a problem arises in that when the operating oil pressure is supplied to both of the hydraulic actuators at the same time, the supply pressure of the operating oil fluctuates due to temporary drop in the overall operating oil pressure. Such fluctuation in the supply pressure of the operating oil is not desirable because the valve opening/closing timings (cam phases) possibly fail to be accurately controlled as a result.